Field of the Disclosure
The present disclosure relates generally to systems for improving communications in cable modem and other systems. More specifically, the present disclosure relates to system and method for reducing interference in ODFM channels.
Related Art
Cable modems (CMs) can be found in both homes and businesses, and are used to transmit and receive digital information (e.g., to access the Internet, view television, and/or view on-demand video, etc.). Numerous CMs can communicate with a device known as a Cable Modem Termination System (CMTS), which is installed at a central location and used to transmit information to CMs, as well as receive information from CMs. The signal between these devices traverses a communications network that includes both coaxial cable and fiber optic cable, and is known as a Hybrid Fiber-Coax (HFC) network or cable “plant.” The HFC allows for bi-directional communication between the CMTS and the CMs. The protocol used to communicate between the CMTS and CMs has been standardized by the CableLabs organization and is collectively known as DOCSIS (Data Over Cable Service Interface Specifications). The set of DOCSIS specifications define all levels of communication including the physical layer, media access control layer, and an application interface layer.
Typically, many CMs share the bandwidth of a single coaxial cable, which usually has a bandwidth of approximately 1 GHz. The 1 GHz spectrum is divided into multiple channels. Each defined channel is typically shared by many CMs. In the downstream direction, from the CMTS to the CM, the CMTS will use time division multiplexing to send data to all CMs using a unique address to send data to a unique CM. In the upstream direction, from the CMs to the CMTS, many CMs must share the same channel. To accomplish this, the CMTS schedules time slots for each CM known as “MAPs.” A given CM is only allowed to send data during its assigned time slot and assigned frequency mini slots. Synchronization signals from the CMTS to the CM keep the different CMs synchronized.
The HFC plant is subject to many different types of impairments that can degrade the quality of the signal. This is especially true in the upstream direction, where noise contributions from many CMs and households combine. These impairments are typically caused by problems such as loose or corroded connections, unterminated lines, faulty equipment, and other noise caused by sources such as motors and lightning. The DOCSIS specification provides a number of different tools to address the most common types of impairments such as: a variety of quadrature amplitude modulation (QAM) constellations; different channel widths; Reed-Solomon Forward Error Correction (R-S FEC); pre-equalization; interleaving; Advanced Time Division Multiple Access (“ATDMA”) (DOCSIS 3.0); and Orthogonal Frequency Division Multiplexing (“OFDM”) (DOCSIS 3.1). By manually varying these parameters, a cable operator can seek to improve signal quality, making tradeoffs between throughput and improved noise immunity.
DOCSIS 3.1 is the new standard for Data-Over-Cable-Service. OFDM technology is first implemented in cable data transfer. During the conversion from DOCSIS 3.0 to DOCSIS 3.1, OFDM and ATDMA signals may exist in the same plant for backward compatibility. The need to support a DOCSIS 3.0 modem will last for many years. Both theoretical simulations and field tests show that once the OFDM fast Fourier transform is performed on the combined signals, the ATDMA signal will have significant spectral spread to each side of the signal in the frequency domain due to a rectangular window function being applied to the OFDM fast Fourier transform function. This will cause a spectral region of 8-10 MHz on each side of the ATDMA signal to be unusable by OFDM carriers, which is unacceptable.
For example, FIG. 1 illustrates a prior art version of the current system. RF signal 10 is received by the analog-to-digital converter 12. The analog-to-digital converter 12 outputs the signal to a plurality ATDMA channel processors 14a-14n. Each of the ATDMA channel processors 14a-14n are identical in the signal processing methods that are employed. The output of the analog-to-digital converter 12 first goes to mixers 16a-16n to shift the signal to a common known frequency, which moves the selected ATDMA channels to a baseband. The outputs from the mixers 16a-16n are then received by filters 18a-18n to recover the ATDMA signal from either combined signals or adjacent ATDMA signals. The clean ATDMA outputs from filters 18a-18n are then received by modules 20a-20n for timing and carrier recovery. The outputs are then received by time domain equalizers 22a-22n for reconstructing the QAM signal. Finally, the outputs from equalizers 22a-22n are received by slicers 24a-24n for eliminating a portion of the signal to obtain the output ATDMA signals 26a-26n. 
The analog-to-digital converter 12 also outputs the signal to a OFDM channel process 30. The output of the analog-to-digital converter 12 first goes to a mixer 32 to shift the signal to a common known frequency, which moves the whole OFDM channel to baseband. The baseband channel is up to 95 MHz in bandwidth in DOCSIS 3.1 upstream and up to 190 MHz in bandwidth in DOCSIS 3.1 downstream. The output from the mixer 32 is then received by a filter 34 to obtain a clean OFDM signal from combined signals or OFDM only signals. The output from the filter 34 is then received by a module 36 for fast Fourier transformation. The output is then received by an equalizer 38 for adjusting the amplitude and reconstructing the signal. Finally, the output from the equalizer 38 is received by a slicer 40 for eliminating a portion of the signal to obtain the output OFDM signal 42. However, the output OFDM signal 42 and the output ATDMA signal 26 may exist in the same plant, and as such, may interfere with each other. Therefore, there exists a need to improve the signal processing in these systems, so that the presence of ATDMA and OFDM signals together do not result in interference and decreased performance.